The separation of low molecular weight species is an extremely important and large volume operation in the chemical and petrochemical industries—particularly in the production of ethylene and propylene. Steam cracking and catalytic cracking are among the largest industrial processes that produce ethylene and propylene. The oxygenate to olefins (OTO) process is another potential source of these streams. All of the above-mentioned processes require recovery and purification of ethylene and propylene to meet stringent product quality specifications. There are some byproducts that are more prevalent in an oxygenates to olefin plant than in a steam or catalytic cracking process. These include methanol, ethanol and dimethyl ether. The methanol, ethanol and dimethyl ether are often present in a stream with other low molecular weight hydrocarbons and oxygenates. The close proximity in boiling points between the various components in the effluent stream makes their separation by distillation expensive and difficult. Thus, there is a need to find alternative means for selectively recovering methanol, ethanol, and dimethyl ether from a C3+ hydrocarbon stream in a more energy-efficient and cost-effective manner. The majority of the C3+ hydrocarbons in the effluent stream of a methanol to olefins plant are propane, butanes and butenes.
Some of the leading alternative separation techniques to distillation involve the use of porous adsorbents that exploit their ability to selectively adsorb some of the components from a mixture. This has given rise to various forms of pressure or temperature swing adsorption (PSA/TSA) processes in which the mixture is first contacted with an adsorbent material under conditions where one or more of the components are selectively removed. The loaded material is then typically exposed to a lower pressure and/or higher temperature environment where the adsorbed components are released and recovered at a higher purity level. Economic viability requires adsorbent materials that can deliver high selectivity, high adsorption capacity, and short duration cycles. An additional and critically important requirement is that the adsorbent material should not catalyze or participate in chemical reactions that might lower the recovery of the desired components and/or render the adsorbent inactive.
There are at least four general categories of porous materials that have been proposed for applications in adsorption-based separation processes. They include ion exchange resins, mesoporous solids, activated carbons, and zeolites. Ion exchange resins and mesoporous solids usually exploit equilibrium adsorption properties in which one or more of the components are selectively adsorbed over suitably dispersed chemical agents. They principally rely on the adsorption affinity of cationic active centers such as Ag and Cu ions for the double bond in the olefins (e.g., π-complexation of propylene). Since these materials rely on adsorption equilibrium properties to effect the separation, the diffusion rates of the various components within the adsorbent do not influence the selectivity of the separation process. Rapid diffusion of the species in and out of the adsorbent material is, however, desirable in order to speed up the contacting of the species with the adsorption sites, leading to adsorption/desorption cycles that have a short duration. Since the pore sizes in these materials are relatively large compared to molecular dimensions, diffusion is generally fast. Thus, the characteristic time associated with the adsorption/desorption cycle is largely controlled by the time required to bring the mixture into thermodynamic equilibrium with the adsorbent. On the other hand, in addition to the basic requirement of adsorption affinity, activated carbons and zeolites can further improve the effectiveness of the separation process by controlling the rates at which molecules diffuse in and out of the material. The diffusional effects in these cases, which are exploited advantageously, are a consequence of the small pore sizes, of molecular dimensions, that make up these high surface area carbons and zeolites. Two limiting cases of diffusion control are frequently exploited for applications in separation. In one extreme case, the separation is achieved by preventing the diffusion of some of the components into the adsorbent. This is generally referred to as separation by size exclusion and can lead to high separation selectivity. The second case exploits a sufficiently large difference in diffusion rates that allows the preferential uptake of some of the components within a predetermined adsorption time. This case is generally known as a kinetic-based separation scheme because the degree of separation depends on the duration of this predetermined adsorption time. Thus, carbons are usually activated to very high surface areas in order to provide textural properties and pore sizes that maximize the number of adsorption sites per unit mass of the material while selectively controlling diffusional transport in and out of the structure. In many applications, zeolites have become even more attractive than activated carbons because of the ever-increasing possibilities afforded by new synthetic routes, which allow for a more flexible and precise control of chemical composition and structure. Whereas chemical composition is used primarily for controlling adsorption affinity, structural properties are used for controlling diffusion rates. The tetrahedrally coordinated atoms in these microporous crystalline materials form ring structures of precise dimensions that selectively control the diffusional access to the internal pore volume.
Eight-membered ring zeolites, in particular, have been actively investigated for the separation of low molecular weight hydrocarbons because their window sizes are comparable to molecular dimensions and because they can provide high adsorption capacities. A typical example is the Linde type A zeolite, which is characterized by a set of three-dimensional interconnected channels having 8-membered ring window apertures. The effective size of the windows can be controlled by appropriately selecting the type of charge-balancing cations. This has given rise to the potassium (3A), sodium (4A) and calcium (5A) forms, which have nominal window sizes of about 3 Å, 3.8 Å, and 4.3 Å, respectively.
In applications involving zeolites, it is well known that the control of window size is critically important for achieving high separation selectivity. For a given zeolite structure type, the effective size of the windows can, in some cases, be modified by partially blocking or unblocking the windows with pre-selected charge-balancing cations. Care must be taken that the adsorbent material does not have any residual acidity and/or that the charge-balancing cations do not promote or participate in detrimental reactions. These reactions not only lower the recovery of the desired components, but they are also likely to render the adsorbent inactive. The double bonds in the olefins are particularly prone to attack even by mildly acidic sites (e.g., isomerization, oligomerization, polymerization, etc) and this may severely limit the temperature and partial pressures at which the separation process can be carried out. The problems of residual acidity are illustrated, for example, by the work M. Richter et al., “Sieving of n-Butenes by Microporous Silicoaluminophosphates”, J. Chem. Soc. Chem. Commun. 21, 1616–1617 (1993), where a proposal is made for the use of SAPO-17 (ERI) for separating trans-2-butene from 1-butene and cis-2-butene. Their work indicates detrimental catalytic activity with their material even at mild temperatures (395K).
Patent EP-B-572239 discloses a PSA process for separating an alkene, such as propylene, from a mixture comprising said alkene and one or more alkanes by passing the mixture through at least one bed of zeolite 4A at a temperature above 273K to preferentially adsorb said alkene and then desorbing the alkene from the bed. EP-A-943595 describes a similar process in which the zeolite adsorbent is zeolite A having, as its exchangeable cations, about 50% to about 85% of sodium ions, about 15% to about 40% of potassium ions and 0% to about 10% of other ions selected from Group IA ions (other than sodium and potassium), Group IB ions, Group IIA ions, Group IIIA ions, Group IIIB ions and lanthanide ions. These patents illustrate the use of suitably chosen charge-balancing cations for controlling chemical composition and the window sizes of the adsorbents.
Zhu et al., “Shape Selectivity in the Adsorption of Propane/Propene on the All-Silica DD3R”, Chem. Commun. 2453–2454 (1999), reports an example that fits the category of separation by size exclusion. Their adsorption uptake measurements indicate that only propylene is able to access the interior of the DD3R crystallites. The exclusion of propane from the adsorbent interior was suggested as the basis for a very selective adsorption scheme.
U.S. Pat. No. 4,605,787 discloses the use of an adsorption-based process for recovering unreacted methanol that is found as an admixture with methyl tert-butyl ether (MTBE) product resulting from the reaction of a C4 stream (e.g., isobutylene) with an excess of methanol. It proposes the use of small pore, 8-membered rings, zeolites 3A, 4A, 5A, and chabazite to selectively adsorb the unreacted methanol from the MTBE product and then desorbing and recycling the methanol by passing the C4 feed stream of the MTBE process as a purge at elevated temperature. The claimed selective adsorption of the methanol by the proposed adsorbents and the recovery of the substantially methanol-free MTBE product suggests that the mechanism of separation is by size exclusion. Such mechanism is also consistent with the sizes of the methanol and MTBE molecules in relationship with the pore sizes of the proposed adsorbents.
U.S. Pat. No. 6,488,741 discloses the use of small pore, 8-membered ring, materials for the kinetic-based separation of propylene from propane. Sorption uptake measurements of propylene and propane on pure silica materials such as CHA, ITE, and ZSM-58 indicate that propylene diffuses much more rapidly than propane and this large difference in diffusion rates is used as a basis for a kinetic-based separation scheme in which propylene and propane mixtures can be separated into their individual components with a high degree of selectivity.
The rate of diffusion of a gaseous species in a porous crystalline material is conveniently characterized in terms of its diffusion time constant, D/r2 (s-1), wherein D is the Fickian diffusion coefficient (cm2/sec) and r is the radius of the crystallites (cm) characterizing the diffusion distance. In situations where the crystals are not of uniform size and geometry, r represents a mean radius representative of their corresponding distributions. The required diffusion time constants can be derived from standard sorption uptake kinetics measurements as described, for example, by J. Crank in “The Mathematics of Diffusion”, 2nd Ed., Oxford University Press, Great Britain, 1975 or by frequency response methods as described, for example, by Reyes et al. in “Frequency Modulation Methods for Diffusion and Adsorption Measurements in Porous Solids”, J. Phys. Chem. B. 101, pages 614–622, 1997.
As noted, there is a need for new, more energy-efficient, adsorption-based methods for selectively recovering methanol, ethanol and/or dimethyl ether from a C3+ hydrocarbon stream. Suitable adsorbents for this application are those having no residual acidity, having high adsorption capacities, and which can be operated in adsorption/desorption cycles of short duration. Short cycles are important for achieving high throughputs that are economically viable. These requirements are well satisfied by the materials and processes of the present invention described below.